Subversion Repositories programming

/*******************************************************************************

 * File: P2_Packet.java

 * Author: Ira W. Snyder (devel@irasnyder.com)

 * Class: CS380 - Computer Networking

 *

 * Assignment: Project #2

 * Date Last Modified: 2006-02-06

 *

 * Purpose: Create and Decode packets for the P2_Server and P2_Client.

 * ****************************************************************************/

class P2_Packet

{

    private static final byte packetBegin = (byte)0xaa; // 0b10101010

    private static final byte packetEnd   = (byte)0x55; // 0b01010101

    private byte packetType;

    private byte packetNumber;

    private byte packetDataSize;

    private byte[] packetData;

    private static byte lastPacketNum = -1;

    /**

     * Packet Format:

     * 1 byte: packetBegin

     * 1 byte: packetType

     * 1 byte: packetNumber

     * 1 byte: packetDataSize

     * packetDataSize bytes: data

     * 1 byte: packetEnd

     */

    /**

     * Packet Types:

     * 0: DROP packet, data contains a single byte telling how often to drop

     * 1: TRANSLATE packet, data contains 4 bytes to translate to 5B encoding

     * 2: REPLY packet, data contains 5 bytes of 5B encoded data

     * 3: QUIT packet, data contains 0 bytes of data

     */

    /**

     * Method: P2_Packet constructor

     * Purpose: Construct a P2_Packet given the type and data

     */

    public P2_Packet (byte type, byte[] data)

    {

        byte[] p = P2_Packet.createPacket(type, data);

        P2_Packet pp = new P2_Packet (p);

        packetType = pp.packetType;

        packetNumber = pp.packetNumber;

        packetDataSize = pp.packetDataSize;

        packetData = pp.packetData;

    }

    /**

     * Method: P2_Packet constructor

     * Purpose: Construct a P2_Packet from a byte[].

     * Note: Checks to make sure the byte[] is a valid packet.

     */

    public P2_Packet (byte[] bytes)

    {

        int i;

        if (bytes.length < 5)

            throw new IllegalArgumentException(); // Definitely a bad packet

        if (bytes[0] != packetBegin)

            throw new IllegalArgumentException(); // Definitely a bad packet

        packetType     = bytes[1];

        packetNumber   = bytes[2];

        packetDataSize = bytes[3];

        packetData = new byte[packetDataSize];

        for (i=0; i<packetDataSize; i++)

            packetData[i] = bytes[i+4];

        if (bytes[i+4] != packetEnd)

            throw new IllegalArgumentException(); // Definitely a bad packet

    }

    /**

     * Method: createPacket()

     * Purpose: Create a packet suitable for transmission over the network,

     * using the current values of this class.

     */

    public byte[] createPacket ()

    {

        return P2_Packet.createPacket (packetType, packetData);

    }

    /**

     * Method: createPacket()

     * Purpose: Create a packet suitable for sending over the network,

     * using the parameters passed in.

     */

    public static byte[] createPacket (byte type, byte[] data)

    {

        int i;

        byte[] packet = new byte[data.length+5];

        packet[0] = packetBegin;

        packet[1] = type;

        packet[2] = ++lastPacketNum;

        packet[3] = (byte)data.length;

        for (i=0; i<data.length; i++)

            packet[i+4] = data[i];

        packet[i+4] = packetEnd;

        return packet;

    }

    /**

     * Method: toString()

     * Purpose: Convert a P2_Packet into a string for printing by a

     * PrintStream object.

     */

    public String toString ()

    {

        int i;

        String s;

        s = new String (    "packetBegin: " + packetBegin + "\n");

        s = new String (s + "packetType: " + packetType + "\n");

        s = new String (s + "packetNumber: " + packetNumber + "\n");

        s = new String (s + "packetDataSize: " + packetDataSize + "\n");

        s = new String (s + "packetData: ");

        for (i=0; i<packetDataSize; i++)

            s = new String (s + packetData[i]+ " ");

        s = new String (s + "\n");

        s = new String (s + "packetEnd: " + packetEnd);

        return s;

    }

    /**

     * Method: getter methods

     * Purpose: get access to private fields

     */

    public byte getPacketType () { return packetType; }

    public byte getPacketNumber () { return packetNumber; }

    public byte getPacketDataSize () { return packetDataSize; }

    public byte[] getPacketData () { return packetData; }

}